The present invention relates to a sewing machine provided with a thread cutting unit. More particularly, the present invention relates to the sewing machine in which a minimized or a reduced length of a leading end portion of the needle thread remains on a back side of a workpiece when a new sewing operation is initiated. The present invention also relates to a method for minimizing a length of the leading end portion of the needle thread between an eye of the sewing needle to an exact cutting end point thereof.
According to a conventional sewing machine having a thread cutting unit, a thread drawn from the lastly stitched point of a workpiece or a fabric is required to be cut when the sewing operation is terminated. For cutting the thread, Japanese Patent Application Kokai No. 56-139793 discloses a thread cutting mechanism in which upon completion of the sewing operation, a movable blade is brought into engagement with a stationary blade by driving a rotating hook shaft for automatically cutting the needle thread and a bobbin thread at a position between a throat plate and a shuttle race complete.
In a sewing machine provided with such thread cutting mechanism, if the needle thread length between a needle eye and the exact cutting point is insufficiently short, the needle thread may be easily disengaged from the eye of the needle at a start phase of subsequent or next sewing operation. That is, thread cast-off may occur. Therefore, in the disclosed device, the thread cutting is performed so that sufficient residual thread length can be provided between the needle eye and the exact cutting point in order to retain the thread to be still engaged with the needle eye at the start timing of the subsequent sewing operation.
On the other hand, conventionally, a thread tension regulating mechanism is provided so as to impart a proper tension to the needle thread positioned between a needle thread supply reel and a thread take up lever. The thread tension regulating mechanism, however, performs tension release operation with respect to the needle thread at several timings, such as ascending timing of a pressure foot for installing or removing a workpiece on or from a sewing machine table, and needle thread cutting timing by the movable blade and the stationary blade of the thread cutting mechanism while the movable blade pulls out the needle thread by a predetermined length.
Particularly, in the sewing machine having the thread cutting mechanism, when the thread cutting operation is completed for completion of the sewing operation, the sewing needle is positioned at its uppermost position. However, this timing is slightly earlier than a timing where the needle thread is pulled to the most upward position by the thread take up lever. Therefore, it is necessary to cut the needle thread so as to remain a relatively large length of the needle thread between the eye of the needle and the exact cutting point of the thread. That is, at an initial phase of the next sewing operation, the thread take up lever is slightly moved upwardly, and further, the needle thread loop must be trapped or defined by the shuttle race complete until the needle thread is pressed by the pressure foot through the workpiece feeding. To this effect, a relatively large length of the leading end portion of the needle thread must be required between the eye of the needle and the exact cutting point capable of providing engagements or several knots between the needle thread and the bobbin thread while still avoiding thread cast off or thread disengagement from the eye of the sewing needle.
In this connection, in the conventional sewing machine having the thread cutting mechanism disclosed in the Japanese Patent Application Kokai No. 56-139793, the thread cast off is avoidable at the initial phase of the new sewing operation. However, since excessive length of the leading end portion of the needle thread is provided, as shown in FIG. 15, the leading end portion of the needle thread Ns remains on the back side of a workpiece W by a large length, and the leading end portion may be entangled with the bobbin thread Bs in the subsequent stitchings to form nest-like thread entanglements. As a result, stitching quality may be lowered.
In another aspect, a thread holding device has been proposed in Japanese Patent Application Kokai No. 61-157487 which device avoids the needle thread cast-off from the needle eye at a first stitching, and also avoids entanglement of the needle thread and the bobbin thread at the back surface of the workpiece. In order to attain these objects, the disclosed device is provided with a thread holding unit and a thread trapping member. An end portion of the needle thread cut by a thread cutting mechanism at the terminal phase of the sewing operation is trapped by the thread trapping member at a position above the throat plate, and the trapped cut end portion of the needle thread is held by the thread holding unit in cooperation with the thread trapping member at a position largely spaced away from the sewing needle.
In the conventional thread holding unit, a nest like thread entanglement is avoidable between the needle thread Ns and the bobbin thread Bs at the back surface of the workpiece W. However, in this conventional device, since the needle thread length between the needle eye and the exact cutting end portion is relatively long and the needle thread Ns is held at a position greatly remote from the needle eye, relatively long needle thread may remain on the top surface of the workpiece W as shown in FIG. 16. Therefore, thread pinching may be required in which the leading end portion of the needle thread is manually pinched and cut. This leads to the consumption of the needle thread which is not used for the exact stitching, and even after the pinching, the remaining cut end portion still remains on the front surface of the workpiece.